A conventional radiology installation principally comprises an X-ray source emitting a wide beam in the direction of a receiver, which may be a film or a brightness intensifier, situated at a definite distance from the source. The structure which is to be examined is positioned between the source and this receiver.
The image obtained is representative of the absorption of the radiation in the structure examined, but the contrasts are restricted by an interference effect, namely X-ray diffusion. This does not constitute a significant absorption, but in a deflection of particular rays, which may occur within the structure which is to be examined as well as within a brightness intensifier, if the installation comprises one. The X-rays are normally propagated in a straight line except in the case of interaction of diffusion at a given point of the path; this point then acts as a secondary transmission source. In other words, the diffusion effect has the result that particular points of the photoluminescent detector receive complementary photons which are deficient from other points, thus impairing the clearness of the image.
To act against this effect, an anti-diffuser grid may be placed between the structure which is to be examined and the brigtness intensifier, this grid having the effect of absorbing a part of the radiation diffused in the said structure.
A more effective method is that referred to as "traveling apertures". The radiological installation is complemented by interposing two diaphragms comprising apertures in the form of a slot, the said slots being arranged and constantly maintained in mutual homothetic relationship. One of the diaphragms is placed between the source and the structure to be examined, whereas the other is placed between the said structure and the receiver. A relative displacement is created between the slots and the structure to be examined. The image is thus acquired "line by line", each line being unaffected by a diffusion component. A modern tomodensitometer may be utilised in a manner simulating the method of traveling apertures, since the X-ray beam is flat and emitted in the direction of a row of detectors. It is then sufficient to move the structure to be examined with respect the the source-detectors array without causing this array to turn as would normally be the case for acquiring an image by tomodensitometry.
As opposed to these methods which consist in an elimination of the diffusion interference component, another approach is known which consists in trying to pre-establish the diffusive function through all the image points and in correcting this image accordingly. An article disclosing this method was published in issue No. 142 of the "RADIOLOGY" review of January 1982. This method may be effective in eliminating the diffusion component established in the brightness intensifier, but it would be difficult to hope to correct in this manner the diffusion interference component formed in the structure to be examined itself, since the first component is of optical nature linked with the image, whereas the second is a physical diffusion of X-rays as a function of the whole volume which is to be examined. However, this second component is very substantial quantitatively, and may in particular cases be of the same order of magnitude as the component representing the image.
The invention has as an object a method able to establish the pattern of the total or overall diffusion component at all points of the image, that is to say, a pattern obtained from data drawn from the image obtained itself and consequently taking into account the diffusion component originating within the structure to be examined.